Studies On Preparation, Micellization And Drug Delivery Properties Of P(CL-LA)-b-mPEG Amphiphilic Block Copolymers

It is well known that when amphiphilic block copolymers are dissolved in a solvent selective for one of the blocks,micelles are usually formed with a rather dense core of the insoluble blocks,surrounded by the diffuse outer corona formed from the soluble blocks. Due to the unique structure and properties,block copolymer micelles become a hot subject as drug delivery vehicles in biomaterial science.This dissertation describes the studies on the preparation,self-assembly behavior and drug release properties of the biodegradable amphiphilic block copolymers consisting of methoxy poly(ethylene glycol)(mPEG)as a hydrophilic block and either crystalline poly(caprolactone-b-L-lactide)(P(CL-LLA))or amorphous poly(caprolactone-b-D,Llactide) (P(CL-DLLA))as a hydrophobic block.The copolymers and micelles are characterized with special attention given to properties which are of most interest to applications in drug delivery such as copolymer composition,thermal behavior,critical micelle concentration,partition coefficient of hydrophobic probes,morphology of micelles and in vitro drug release.Investigations are focused on the effects of the crystalline nature of core-forming blocks on the formation of the micelles in H₂O, especially on micellar morphologies,the drug loading and releasing properties.Specifically,five sections of work were carried out as follows:(1)Amphiphilic block copolymers consisting of methoxy poly(ethylene glycol) (mPEG)and either crystalline poly(caprolactone-b-L-lactide)(P(CL-LLA))or amorphous poly(caprolactone-b-D,L-lactide)(P(CL-DLLA)),respectively,were prepared under mild conditions using dicyclohexylcarbodiimide(DCC)as coupling agent.The composition and structure of the copolymers were characterized by ¹H NMR,¹³C NMR and GPC measurements.(2)Differential scanning calorimetry(DSC)study was carried out to get a perspective on the solid-state thermal properties of the resultant copolymers under a nonisothermal condition.The analysis revealed the crystallization behavior of P(CL-LLA)block is affected significantly by the molecular weight of the conjugated mPEG block,and the glass transition temperature(T_g)of P(CL-DLLA)block is depressed by the existence of mPEG moiety in theblock copolymer.The crystallization behavior was also characterized by the technique of polarized light microscopy(PLM).(3)Micelles were then prepared from the resultant copolymers with predetermined hydrophobic and hydrophilic blocks in H₂O by dialysis method.The self-assembly behavior was investigated by fluorescence measurement.The results proves that the block copolymers all have a strong tendency to form micelles in aqueous medium,with very low critical micelle concentrations(CMCs).The CMC of P(CL-LLA)-b-mPEG is higher than that of P(CL-DLLA)-b-mPEG when mPEG block with the same molecular weight.Furthermore,partition equilibrium coefficient(K_v)of pyrene in the micellar solution of P(CL-LLA)-b-mPEG copolymer was lower than that of P(CL-DLLA)-bmPEG copolymer when mPEG block with the same length.In respect that the similar weight percentage of hydrophobic block for P(CL-LLA)-b-mPEG and P(CL-DLLA)-bmPEG with the same length mPEG block,this depression of K_v values implies the difference in micellar core's physical structure between these two systems,i.e.,the crystalline P(CL-LLA)block and the amorphous P(CL-DLLA)block.The geometrical parameters(the size and morphology)of the micelles were characterized by dynamic light scattering(DLS)and transmission electron microscopy (TEM)techniques.TEM images showed that the micelles made from P(CL-DLLA)-bmPEG copolymers adopt a spherical shape.In contrast,the micelles arising from P(CLLLA) -b-mPEG copolymers exhibit a cylindrical shape.DSC and wide-angle X-ray diffraction(WAXD)methods were also used in an attempt to gain insight into the nature of the micellar core formed by P(CL-LA)diblock copolymers.Based on the confirmation of these analyses,it is believed that the high enthalpy of crystallization of P(CL-LLA)core-forming block combining with its hydrophobicity in aqueous medium is the driving force for the cylindrical morphology.Finally,from thermodynamic point of view,we propose a possible mechanism for the cylindrical morphology of P(CL-LLA)-b-mPEG micelles:a strong competition between the energy of the crystalline core and chain stretching of the densely grafted mPEG corona would be responsible for the formation of cylindrical micelles.(4)Indomethacin(IMC)was chosen as a model drug to study the in vitro drug release behavior from micelles.Drug-loaded micelles were prepared by dialysis and O/W methods,respectively.Fourier transform infrared spectroscopy(FTIR)and DSC techniques were carried out to study the compatibility and interaction between the polymer micelles and drug.The morphology of drug-loaded micelles was characterized with special attention by DLS and TEM measurements.(C-L)-E₅₀and(C-L)-E₂₀IMC-loaded micelles exhibit the same cylindrical morphology as the blank micelles.However,an obvious morphology transition was observed in(C-L)-E₁₁micelles with drug loaded.Except cylindrical micelles,some aggregations with large size appeared.The influence of the copolymer concentration on the micelle morphology was also studied.For P(CL-DLLA)-b-mPEG series,(C-DL)-E₅₀IMC-loaded micelles adopt the same spherical shape as the blank micelles.But in comparison with blank spherical micelles, (C-DL)-E₂₀and(C-DL)-E₁₁IMC-loaded micelles exhibit the morphology of spherical and cylindrical coexistence.Furthermore,as the copolymer concentration decreased,the spherical micelles became the primary morphology,and cylindrical shapes became lessened.It is believed that the solubilization of hydrophobic and crystalline IMC into the micellar cores would affect the surface tension in the core-water surface and then make the surface free energy altered.Once the hydrophilic mPEG block is too short to stabilize the whole micelle,a transition from a sphere to a cylindrical morphology would be preferred as a equilibrium structure from thermodynamic point of view.In addition,the morphology stability of drug-loaded micelles in water was also studied.(5)IMC-loaded micelle solutions were prepared by dialysis and O/W methods, respectively.Ultraviolet(UV)spectroscopy measurement was used to characterized the initial amount of IMC loaded in micelle solutions and the in vitro release profile.The results reveal that the preparation method of drug-loaded micelles can affect the incorporation process of drug into micelles.The whole process of IMC release from micelles should including the following processes:(a)H₂O molecules penetrate into inner micelle and make the cores swelled;(b)drug molecules dissolve in water and diffuse from inner core→corona→water;(c)the drug diffusion process would influence the physical structure of micellar cores,i.e.the exposure of hydrophobic blocks in water maybe induce the shift of micelle thermodynamic and dynamic equilibrium; The release mechanism of IMC from drug-loaded micelles prepared by dialysis and O/W methods was fitted by different drug release kinetic models.In detail,the drug release rate would be controlled by such factors as follows:(1)the initial drug amount in micelles;(2)the interactions between the drug and micellar core;(3)the physical state of micellar cores;(4)the distribution of drug in micells;The influence of mPEG corona-forming block length on the drug release rate was also investigated.Finally,the micelle morphology after IMC has been released for 192hr was characterized by TEM.And the morphology transition confirms the effects of drug release process on the physical state of micellar cores as well as thermodynamic and dynamic equilibrium in the micelle systems.A possible mechanism about the morphology transition in this process was proposed.